The present invention relates to methods and apparatus for processing a wet web of fiber material with a cellulose content, preferably a soft paper web, so as to imprint the web to impart a desired surface structure to the web and to dry the web, and for reeling the imprinted and dried web into a roll.
In conventional production of paper, a continuous web is formed from a suspension of pulp delivered onto a moving wire, and the web with originally a very high water content is partially dewatered on a wire to form a wet web with at least some degree of coherence, although the fibers are still capable of undergoing rearrangement at this stage of the paper making process. The wet web is then fed to a press section, comprising one or several wet presses each having at least one roll nip through which the web runs, and in which roll nip still more water is mechanically pressed out of the web. The web is then made to pass through a drying section in which remaining water is evaporated to a predetermined dry solids content for the web through a supply of heat. Finally, the web is reeled up into a finished material reel or roll in a reel-up. At all web transfers between two consecutive machine sections, the web must transferred from one drive section to the next over an open or closed draw. If the web in the draw lacks support from a belt or the like then the draw is termed an open draw. At the open draw the web must bear its own weight and also be able to tolerate increased tensions caused by speed differences between the drive sections in question. The risk for web breakage in an open draw is high and therefore open draws should be avoided.
In recent years press sections have been developed that have made it possible to transfer the web from the forming section of the paper machine through all roll nips without any considerable tension on the wet paper web. Through the gradual increase of the dry solids content of the web, the strength of the web grows continually during its passage through the press section. Nevertheless, the web still is sensitive to the increased tensions, which occur at transfers between different sections of production. Furthermore, in a conventional wet press, rewetting of the web tends to occur at the outlet of every roll nip, which negates some of the gained strength of the web.
Accordingly, it would be desirable to provide a higher dry solids content of the wet web at an earlier stage of the production process. It would also be desirable to eliminate transfers and open draws throughout the pressing, drying, and reeling operations of a papermaking process, which would facilitate a higher web velocity and thereby a substantial increase in production with subsequent benefits for the paper industry. It would also be desirable to provide such benefits while also providing a higher bulk for soft paper and better quality in the final product.
Furthermore, in production of paper, board, and cardboard, currently it is common to have a very elongated drying section with a great number of drying cylinders and guide rolls that alternate with each other, around which the wet web runs during heating and evaporation of water. In production of soft paper, which has lower grammages and which for instance is used for production of household paper, paper towels and other hygiene products, it is more typical to employ a drying section that comprises either a single drying cylinder with a relatively large diameter (e.g., up to 6 m in diameter) and with a polished mantle surface that is steam heated from its inner side. i.e. a so called Yankee dryer, or a combined machine which consists of both a Yankee dryer and a number of common drying cylinders in succession. Consequently, these machines employing a large number of drying cylinders tend to be quite long.
However, attempts have been made to reduce one or several of the problems mentioned above. For instance, from U.S. Pat. No. 4,324,613 it is known that pressing of a wet web in a hot press simultaneously at increased temperature and high pressure leads to several favorable effects, e.g., a considerably increased dewatering speed and thereby a shortened drying section. The term xe2x80x9chot pressxe2x80x9d which is used in the present context and which constitutes an integration between a press and a drying arrangement is improperly named by many an xe2x80x9cimpulse dryer.xe2x80x9d The patent describes a roll nip that is formed between two cylindrical rolls movable towards each other, one of the rolls being heated from the outside to a high surface temperature. The simultaneously applied high temperature and high-pressure give a high thermal flow to the web, which results in a high dewatering speed. The good dewatering most likely results in part from the considerable steam generation at the roll nip, which helps to blow or force remaining liquid out of the web, and from the increased temperature that reduces the water viscosity so that it is more easily pressed out of the web. Additionally, the water, which during the exertion of the roll nip pressure attains a temperature in excess of 100xc2x0 C., rapidly evaporates when the pressure rapidly drops as the web exits the roll nip.
However, in a conventional roll nip the dwell time for the paper web is too short in order to get any real benefits from the favorable effects of the hot pressing. However, from U.S. Pat. No. 5,393,384 it is known to employ a shoe press with a concave press shoe that acts against a heated drying cylinder and forms an elongated nip through which the web passes. Since the nip of the shoe press is extended to approximately 20 to 30 cm, the dwell time of the web in the nip is substantially extended relative to a roll press, so that the web is subjected to high pressure and temperature for a much longer period of time.
Furthermore, when producing soft paper such as tissue, there is generally a desire to attain a web with a high bulk (i.e., ratio of volume to weight), since high bulk tends to provide both softness and high power of absorption.
There have been many attempts to manufacture soft paper with substantially increased bulk. For instance. U.S. Pat. No. 3,806,406 describes a method and a device for forming of a soft paper web with high bulk. A wet web is fed on a felt through a roll nip comprising a press roll and a heated Yankee dryer. Because the pressing of the web tends to reduce the bulk, the mantle surface of the Yankee cylinder comprises alternately raised and lowered surface portions, which constitutes a relief pattern for placing against the web. Thus, only those parts of the web in contact with the raised surface portions are compressed, while the parts situated in between arc left relatively unaffected. The entire drying is done thermally through the supply of heat from the Yankee dryer, during which the soft paper web is held in a fixed position in relation to the raised and lowered surface portions. Accordingly, the web acquires a certain texture, i.e., a predetermined pattern of compressed and uncompressed surface portions, which surface portions match the relief pattern of the mantle surface. The web thereby also obtains an increased bulk.
However, the Yankee dryer has some disadvantages. One problem, as mentioned above, is that the Yankee dryer has a large diameter and therefore is very voluminous. Another problem is that when using a Yankee dryer it is difficult to achieve as high a temperature as can be achieved in hot pressing. Despite the fact that the steam within the drum may have a very high temperature, the drum mantle surface typically only reaches approximately 95xc2x0 C. to 100xc2x0 C. The mantle of the Yankee dryer must have a considerable thickness, typically approximately 50 to 100 mm, in order to be able to resist both the internal steam pressure and the external pressure against the mantle surface. The dryer thus represents a very large mass of material that must be heated up, and the thick mantle effectively blocks the thermal flow from the drum interior to the exterior mantle surface, leading to high energy consumption and attendant high cost of operation. Another problem is that it is difficult to maintain the desired geometrical shape of the roll in the cross machine direction on an account of difficulties in maintaining the same temperature along the mantle surface of the roll and on the gables. Since the mantle surface and the gables cannot expand freely and independently of each other, large tensions occur between these components, and hence the mantle surface often tends to become arc-shaped, outwardly or inwardly, across the machine direction.
U.S. Pat. No. 5,830,316 discloses an apparatus and method for drying and imprinting a wet web in which the web is passed through a wet press nip in which the web receives an imprinted structure. The web is then moved with the imprinted structure substantially maintained to a drying section in which the web is dried, which drying section can comprise a through-blow dryer and an Yankee cylinder, from the latter of which the dried web must be creped. There is an unavoidable open draw downstream of the creping doctor before the web can be reeled up, which is undesirable because of the increased risk of web breakage. Furthermore, the wet press tends to rewet the web.
Another process for drying a soft paper web is described in U.S. Pat. No. 5,556,511. A soft paper web is moved together with a felt for absorption of liquid from the web through a press zone, and the web is heated before or inside the press zone. The sudden expansion of the steam in the web when said web leaves the press zone is said to cause a delamination, which improves the volume and the softness in the web. i.e., increases the bulk of the web. Still more thermal drying can be performed after the web leaves the press zone by means of further drying devices such as a drying cylinder. Since the drying cylinder has a smooth mantle surface, to which the web easily sticks, the dried web is detached by creping from the drying cylinder by means of a creping blade. A drawback of this process is that the creped structure of the web tends to be lost after the web has absorbed water. Another disadvantage is that an unavoidable open draw is created from the creping blade to the reel-up, across which the web must be transferred before any final reeling can be done.
The above needs are met and other advantages are achieved by the present invention, which provides an apparatus and method for imprinting, drying, and reeling a fibrous web that enables a substantial reduction in the total machine length eliminates the remaining transfers and open draws for the web between the press section and the completed paper roll formed in the reel-up of the machine, and at least substantially reduces the problems of rewetting of the web in the press section, high heating costs, and poor retention of the desired structure of the imprinted web upon wetting of the dried web.
In accordance with a preferred embodiment of the invention, a method for imprinting, drying, and reeling a web includes the steps of passing the web through a nip defined between a pair of press members with one side of the web in contact with a surface of a clothing that also passes through the nip, and passing a heat transfer device through the nip along with the web and clothing. The heat transfer device defines a heatable surface in contact with the opposite side of the web during the web""s passage through the nip, and the surface of the clothing, or the heatable surface of the heat transfer device, or both, comprises an imprinting surface for imprinting the web when the web is pressed in the nip. The method further includes the steps of heating the heat transfer device such that the heat transfer device heats and dries the web during contact therewith, supporting the web on the heat transfer device as the heat transfer device passes through the nip and travels a path from the nip to a rotatable reel spool or core shaft onto which the web is reeled to form a finished roll of web material, and transferring the web from the heat transfer device onto the reel spool at a location proximate thereto such that the web is supported by the heat transfer device along substantially the entire path from the nip to the reel spool. Preferably, the reeling of the web is performed with the reel spool against the heat transfer device such that the web is transferred directly from the heat transfer device onto the reel spool. Accordingly hot pressing of the web in the nip substantially reduces the problem of rewetting of the web upon exiting the nip and achieves a high degree of dewatering of the web. Moreover, the machine arrangement becomes entirely free from open draws and transfers from the press section to the finished material reel, and the machine arrangement can be made very compact.
An apparatus for imprinting, drying, and reeling a fibrous web in accordance with a preferred embodiment of the invention comprises a hot press including a pair of press members forming a nip therebetween for passage of the web therethrough and at least one clothing arranged to pass through the nip of the press with one side of the web in contact with a surface of the at least one clothing. The apparatus also includes a heat transfer device defining a heatable surface, the heat transfer device being arranged to pass through the nip of the press with the opposite side of the web in contact with the heatable surface, and a heating device operable to heat the heat transfer device such that the heat transfer device heats and dries the web during contact therewith. The apparatus further includes a reel-up including a rotatable reel spool or core shaft onto which the web is reeled to form a finished roll of web material. At least one of the clothing surface and the heatable surface comprises an imprinting surface for imparting a textured surface to the side of the web in contact therewith when the web, clothing, and heat transfer device are passed through the nip of the hot press. The heat transfer device is arranged to support the web from the press substantially to the reel spool.
Various configurations can be used for the heat transfer device. In one embodiment, a first of the press members of the hot press comprises a surface winding drum having a generally cylindrical outer surface and being rotatable about a central axis thereof, and the heat transfer device and the surface winding drum comprise one and the same member. Thus, the generally cylindrical outer surface of the surface winding drum comprises the heatable surface that is heated by the heating device, and the reel spool of the reel-up is disposed proximate the surface winding drum for receiving the web therefrom after the web""s passage through the nip. Preferably, the reel spool is against the Web on the surface winding drum such that the reeling of the web is performed against the surface winding drum.
The generally cylindrical outer surface of the surface winding drum can comprise an imprinting surface for imprinting the side of the web in contact therewith. Additionally or alternatively, the at least one clothing can include an imprinting fabric in contact With the web for imprinting the side of the web in contact therewith as the web and imprinting fabric pass through the nip. Where the imprinting fabric is used, the surface of the surface winding, drum can be smooth or can have an imprinting pattern. Thus, either or both sides of the web can be imprinted in the hot press. Generally, the at least one clothing will also include an absorbent fabric for absorbing water pressed from the web as the web and absorbent fabric pass through the nip.
The heating device preferably includes at least a first heater disposed proximate the outer surface of the surface winding drum for heating the drum surface ahead of the nip. The heating device can also include at least a second heater disposed proximate the outer surface of the surface winding drum after the nip for healing the web carried on the drum surface.
In accordance with another embodiment of the invention, a first of the press members comprises a generally cylindrical roll rotatable about a central axis thereof, and the heat transfer device is formed separately from the rotatable roll and is configured as a closed loop loosely encircling the rotatable roll, the roll frictionally engaging the heat transfer device at the nip so as to rotatably drive the heat transfer device. The heat transfer device in one preferred embodiment comprises a substantially rigid sleeve. By substantially rigid is meant that the sleeve has sufficient rigidity to maintain its desired round shape during use even though the sleeve is substantially unsupported by the roll over most of the sleeve""s circumference. Preferably, the sleeve forms a nip with the reel spool about which the web is wound in the reel-up. Thus, reeling of the web is performed directly against the sleeve.
An outer surface of the sleeve that contacts the web can comprise an imprinting surface for imprinting the web. Additionally or alternatively, the at least one clothing of the hot press can include an imprinting fabric in contact with the web in the nip for imprinting the web. Thus, either or both sides of the web can be imprinted.
In another preferred embodiment of the invention, the heat transfer device comprises a flexible belt looped about the rotatable roll and about at least one additional guide roll. The outer surface of the belt that contacts the web can comprise an imprinting surface for imprinting the web, and/or the at least one clothing can include an imprinting fabric in contact with the web in the nip for imprinting the web, whereby either or both sides of the web can be imprinted. The apparatus can also include a blowing device disposed proximate the reel-up for blowing a gaseous fluid against the web to aid in separating the web from the belt so that the web can be wound onto the reel spool. The belt can be permeable and the blowing device can be disposed inside the loop of the belt.
In accordance with another embodiment of the invention, a first of the press members of the hot press comprises a generally cylindrical roll rotatable about a central axis thereof and a substantially rigid sleeve loosely encircling the rotatable roll, and the heat transfer device comprises a flexible belt looped about the sleeve and about at least one additional guide roll, the rotatable roll frictionally engaging the sleeve at the nip to rotatably drive the sleeve and the sleeve frictionally engaging the belt to rotatable drive the belt. An outer surface of the belt in contact with the web can comprise an imprinting surface for imprinting the web, and/or the at least one clothing can include an imprinting fabric in contact with the web for imprinting the web, whereby either or both sides of the web can be imprinted.
The present invention facilitates a substantially shortened total machine length compared with existing machine designs. Moreover, the energy consumption of the machine can be reduced relative to conventional machines employing Yankee dryers, through-air dryers, and/or several drying cylinders. The web furthermore is dried without creping, and yet a web with a high bulk is produced by virtue of the imprinting in the hot press, which bulk remains after rewetting of the web, with qualities similar to those obtained by drying the web in a through-air dryer.